May & might

may

We can use 'may' to ask for permission. However this is rather formal and not used very often in modern spoken English.

• May I borrow your pen?

• May we think about it?

• May I go now.

We use 'may' to suggest something is possible.

• It may rain later today.

• I may not have time to do it today.

• Pete may come with us

might

We use 'might' to suggest a small possibility of something. Often we read that 'might' suggests a smaller possibility that 'may', there is in fact little difference and 'might is more usual than 'may' in spoken English.

• She might be at home by now but it's not sure at all.

• It might rain this afternoon.

• I might not have time to go to the shops for you.

• I might not go.

For the past, we use 'might have'.

• He might have tried to call while I was out.

• I might have dropped it in the street.

Should

We use 'should' for giving advice.

• You should speak to him about it.

• He should see a doctor.

• We should ask a lawyer.

We use 'should' to give an opinion or a recommendation.

• He should resign now.

• We should invest more in Asia.

• They should do something about this terrible train service.

 

'Should' expresses a personal opinion and is much weaker and more personal than 'must' or 'have to'. It is often introduced by ' I think'.

• I think they should replace him.

• I don't think they should keep the contract.

• Do you think we should tell her?

Must & Have to

'must' is used to show that we are sure something is true. It is a logical deduction based upon some clear evidence or reason.

• It must be nice to live in Florida where the weather is always good.

• There's no heating on. You must be cold.

• You must be worried about her. She is very late coming home.

• I can't remember what I put it. I must be getting old.

'must' is also used to express a strong obligation. We use 'must' when it is some personal circumstance that makes the obligation necessary (and the speaker almost certainly agrees with the obligation.)

• They must do something about this road. It is very dangerous.

• I must go to bed earlier in future.

• You must come and see us when you next come to England.

• I must say, I don't think you were very polite to him.

We also use 'have to' to express a strong obligation. When we use 'have to' this usually means that some external circumstance makes the obligation necessary.

• We have to put in our offer today or lose out on the contract.

• I have to arrive very early. My boss is very strict.

• You have to get good grades in your exams or the university will not accept you.

• I have to send a report to my manager every week.

In British English, 'have got to' is used in the same way as 'have to'.

• We've got to finish now. Our time is up.

• I've got to take this dress back to the shop and get it changed.

'will have to' is also used to talk about strong obligations. Like 'must' this usually means that that some personal circumstance makes the obligation necessary. (Remember that 'will' is often used to show 'willingness'.)

• We'll have to do something about it.

• You'll have to speak to her.

• We'll have to have lunch and catch up on everything.

• I'll have to get back to you later.

As you can see, the differences between the affirmative forms are sometimes very small and very subtle. However, there is a huge difference in the negative forms.

'mustn't' is used to express strong obligations NOT to do something.

• You mustn't leave your door unlocked. Something will get stolen.

• You mustn't talk about it. It's confidential.

• I mustn't drink. I'm driving tonight.

• You mustn't phone me at work. We aren't allowed personal calls.

'don't have to' (or 'haven't got to' in British English) is used to state that there is NO obligation or necessity, it is a question of choice.

• You don't have to book. You can buy a ticket when we get there.

• I don't have to listen to this. I'm leaving.

• You don't have to do it if you don't want to.

• She doesn't have to sign anything at this stage.

• I haven't got to go just yet. I've got another hour.